The present invention relates to a room temperature-curable silicone rubber composition or, more particularly, to a room temperature-curable silicone rubber composition capable of giving a cured silicone rubber having extremely high resistance against oils.
Needless to say, various kinds of sealing materials such as gaskets and packings are used in and around automobile engines and these sealing materials are usually shaped of a highly oil-resistant material such as cork, organic rubbers, asbestos and the like in view of contacting of oils with the sealing material. These conventional sealing materials, however, are not quite advantageous in respect of management of stockpiles and process control, if not to mention the expensiveness, and are also not very reliable in their sealing performance.
In this regard, these conventional sealing materials are increasingly being replaced with FIPG (formed-in-place gasket) type sealing materials using various kinds of room temperature-curable rubber compositions including room temperature-curable or room temperature-vulcanizable silicone rubber compositions, referred to as RTV silicone rubber compositions hereinbelow, as the most useful ones which are highly evaluated in respect of the workability, sealing performance and heat resistance.
In compliance with the requirement in the modern automobile technology toward decrease of fuel consumption, it is a trend that the multi-grade gear oils of low viscosity used in automobile transmissions are admixed with an extreme pressure additive in a considerably large amount so that the sealing materials made of silicone rubbers are susceptible to the attack of such an oil containing a large amount of the extreme pressure additive at high temperatures leading to eventual leakage of oils after prolonged use of the sealing material.
Various proposals and attempts have been made in this regard of improving the oil resistance of silicone rubber-made sealing materials obtained from a RTV silicone rubber composition. For example, Japanese Patent Kokai 57-76055 teaches an improved formulation of RTV silicone rubber compositions in which finely divided magnesium oxide powder having a specific surface area of at least 50 m2/g is compounded in combination with a reinforcing silica filler. Further, Japanese Patent Kokai 59-80463 teaches a formulation in which zinc carbonate or zinc oxide is compounded with RTV silicone rubber compositions. The improvements so far proposed, however, are still insufficient to provide a complete solution of the problem in respect of the oil resistance of silicone rubbers since the automobile oils in recent years are admixed with a further and further increasing amount of an extreme pressure additive to comply with the requirement of upgrading of oils against which the FIPG sealing materials also should heve further increased resistance. Therefore, it is eagerly desired to develop a novel RTV silicone rubber composition capable of withstanding contacting with such a high-performance automobile oil at high temperatures with a long serviceable life
SUMMARY OF THE INVENTION
The present invention has an object to provide such an improved RTV silicone rubber composition to meet the severest requirement in the automobile technology in respect of the oil resistance, and the RTV silicone rubber composition provided by the invention comprises:
(a) 100 parts by weight of a diorganopolysiloxane having a linear molecular structure and terminated at both molecular chain ends each with a silanolic hydroxy group; PA0 (b) from 1 to 25 parts by weight of an organosilicon compound having, in a molecule, at least two hydrolyzable group bonded to the silicon atom or atoms; and PA0 (c) from 0.1 to 50 parts by weight of an alkali metal salt of a weak acid of which the acid dissociation constant as expressed by pK.sub.a is in the range from 2.0 to 12.0 at 25.degree. C.